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Authors' Reply FARGHALLI A. MOHAMED and PRABIR K. CHAUDHURY In a recent investigation, Ill Chaudhury et al. examined the creep behavior of four grades of Zn-22 pct A1, A, B, C, and three containing 400, 125, 40, and 1 ppm Fe, respectively. The experimental results on grades A, B, and C have shown that the creep behavior of these three grades exhibits a sigmoidal relationship between stress and strain rate which is manifested by the presence of three regions: region I (low stress region), region II (intermediate stress region), and region III (high stress region). In region II, the creep characteristics, including the stress exponent, n, and the activation energy for superplastic flow, Q, are insensitive to Fe level; n ~- 2.5 and Q = Qgb, where Qgb is the activation energy for grain boundary diffusion. By contrast, the results have revealed that region I behavior is affected by Fe level; in general, a higher Fe level gives rise to a higher stress exponent (n > 2.5) and a higher activation energy (Q > Qgb). In addition, the results have shown that when highpurity Zn-22 pet AI (grade 3 containing 1 ppm of Fe) is creep tested, no evidence of region ! at the lowest strain rate used in the investigation (2 • 10 -7 s - l ) is noted. Chaudhury et al. have suggested r~j that the sensitivity of region I to Fe level is most likely a reflection of the presence of a threshold stress for creep in Zn-22 pct A1 whose origin is related to Fe segregation at boundaries. By considering thermodynamic properties for the Zn-22 pct A1 and the free energy change for the reaction between Fe and A1 (Fe + 3 A1 = FeAl3), Tang t21 has concluded that the solubility of Fe in the Al-rich phase of the alloy is insignificant and that, as a result, all Fe in the previous Zn-22 pct Al grades exists in the form of FeAl3 particles. On the basis of his conclusion, Tang t21 has suggested the possibility that the threshold stresses reported previously by Chaudhury et al. ul are related to the interaction between the particles and grain boundary dislocations. There are three comments regarding the possibility raised by Tang. 12] First, analyses based on thermodynamic properties and functions generally involve approximations and assumptions that may significantly influence the numerical values of the predictions. For example, Tang ~2~used the properties of a binary system, METALLURGICAL AND MATERIALS TRANSACTIONS A
Zn-A1, to predict the behavior of a ternary system, Zn-A1-Fe. This may not be appropriate since the activities for Zn and AI in the presence of Fe may be different from those for the two elements in high-purity Zn-22 pet A1. Second, the presence of other trace elements, such as Cu, Mg, Pb, etc., may affect the solubility of Fe in the alloy; this possibility was reported for other metallic systems. Third, threshold stresses reported for Zn-22 pct AI grades containing Fell.3] are very sensitive to temperature, as described by the following equation: IL3-Sj "ro/ G = flo exp(Qo / R T )
[1]
where /30 is a constant and Q0 is an activation energy term
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